The present invention relates to a method and apparatus for cleaning boreholes. In particular, but not exclusively, the present invention relates to a method and apparatus for removing particulate debris from a casing-lined borehole in an onshore or offshore oil or gas well.
It is known to create an onshore or offshore oil or gas well by drilling a borehole extending from the surface (at ground or seabed-level respectively), before installing a cylindrical, typically metal casing in the borehole, and cementing the casing into the borehole. The borehole may be xe2x80x9cdeviatedxe2x80x9d (extending at an angle from the vertical) and may feature branch or lateral boreholes which may themselves be lined and cemented. Such operations often lead to the inside wall of the casing becoming soiled with materials such as drilling mud residue (xe2x80x9cmud-cakexe2x80x9d), well fluid residue, and cement residue, which may hamper subsequent downhole operations, and the satisfactory withdrawal of well fluids.
In order to overcome problems associated with the build-up of such materials, it is necessary to physically remove these materials from the casing wall. Typically this is accomplished by inserting a rotating string having a drill bit and or a dedicated casing scraper tool into the casing, running the drill bit and or scraper to the bottom of the casing, and then working the drill bit and or scraper up and down the casing. The residue materials are then circulated out of the well by pumping a cleaning fluid through the casing, which transports the materials to the surface.
However, it becomes increasingly difficult to circulate the materials out of the casing in extended reach and deviated wells. Therefore a number of devices have been developed to facilitate entrainment and removal of the residue materials, incorporating brushes and other agitators. However, these devices have been found to be unreliable or ineffective in removing the residue materials.
It is amongst the objects of the present invention to obviate or mitigate at least one of the foregoing disadvantages.
According to a first aspect of the present invention, there is provided a method of cleaning a casing-lined borehole, the method comprising the steps of:
circulating fluid in the borehole to entrain material therein;
separating the entrained material from the fluid within the borehole; and
removing the separated material from the borehole.
According to a second aspect of the present invention, there is provided an apparatus for use in removing material from a borehole, the apparatus comprising:
circulating means for circulating fluid in a borehole to entrain material therein; and
separating means for separating the entrained material from the fluid within the borehole.
References to a casing-lined borehole refer to a borehole which has been lined with a suitable casing, liner, or any other suitable tubular lining member, as will be appreciated by persons skilled in the art.
Thus the present invention may allow a fluid to be circulated in a casing-lined borehole to entrain material in the borehole, typically material gathered in the end of the borehole or in the xe2x80x9clowxe2x80x9d side of an inclined or horizontal bore, by entraining the material in a carrier fluid, separating the material from the fluid and subsequently removing the separated material to the surface.
The material may be mud residue, such as mud-cake, well debris, or cement residue or the like, produced by the operations involved in creating a lined borehole. Further, the material may be sand or scale, which may build up in the bore during production. The material may have been adhered to the inner wall of the borehole, and may be dislodged from the borehole inner wall in the course of the cleaning operation. Preferably, the material is dislodged using a drill bit and or a casing scraper coupled to a support string which also supports the apparatus. The string, for example a string of drill pipe, may be rotated from the surface, and may be run into the borehole to the region of the borehole to be cleaned before being moved axially in the borehole to dislodge material from the wall thereof. Alternatively, the apparatus may be run on wireline or coiled tubing.
The fluid may be a viscous mud, a cleaning fluid such as brine, and may contain any appropriate additives. The fluid may be pumped down the borehole from the surface, through a string bore or through an annulus between a string and an inner wall of the borehole, or may be recirculated within the borehole.
Preferably, the circulating means includes an impeller, preferably a screw, which is rotatable to facilitate circulation of fluid in the borehole. The impeller may be coupled to a supporting string, such that rotation of the string imparts rotation on the impeller. Alternatively, the impeller may rotate while the supporting string remains stationary. In other embodiments, for example where the apparatus is mounted on wireline or coiled tubing, rotation may be provided by electric motor or hydraulic motor. The circulating means may further include a pump located on surface or in a supporting string.
Preferably, a tubular member or sleeve is provided, having an inlet for receiving fluid circulating in the borehole. The inlet may be normally closed, and may be opened by fluid pressure force, for example by fluid being pumped through a supporting string. One or more fluid jetting outlets may be provided above the inlet, to permit fluid to be jetted into the annulus above the inlet to create a barrier to carrier fluid flow. Radially extending flow deflectors, which may be in the form of blades, may also be provided above the inlet, to scrape or otherwise dislodge material from the inner wall of the borehole and into the fluid inlet. The flow deflectors may be normally retracted, and may be extended by fluid pressure. Preferably also, the impeller is located within the tubular member. The material may be separated from the fluid within the tubular member. The tubular member may include an outlet having a filter which retains the solid material, allowing the fluid to pass therethrough to return to the surface or to pass to a downhole pump for recirculation. The filter may be annular or cylindrical, or may be formed by forming the outlet of restricted area openings, such as slits. A plurality of such filters may be provided, for example the filters may define successively reducing flow passages. Preferably, the impeller is adapted to clean the filter, for example the impeller may be a screw and move across the face of the filter. This prevents build up of material on the filter, and minimises the possibility of the filter clogging.
The fluid may be pumped into the borehole through a string of pipes passing through the tubular member and having an outlet in the borehole. The fluid outlet may be at or towards the bottom of the string. The outlet may be provided in a drill bit. Alternatively, the fluid may be pumped into the borehole down an annulus formed between an outer wall of the tubular member and the wall of the borehole. In a yet further alternative, the tubular member may be provided on a wireline, slickline, or coil tubing assembly.
A venturi may be disposed within the tubular member to create a restriction to flow of fluid through the tubular member, to increase the fluid velocity and aid circulation of the fluid and entrainment of solid material therein.
One or both of the impeller and tubular member may be coupled to a support string. In one embodiment, a differential gear assembly is provided to couple the tubular member to the support string. Thus, when the string is rotated, which may also serve to dislodge material from the inner wall of the borehole, the tubular member may be counter-rotated. Alternatively, the tubular member may be fixed against rotation within the borehole such that relative rotation between the tubular member and the string may be provided when the string is rotated.
In a further alternative embodiment, the impeller may be coupled to the tubular member. The string may remain rotationally stationary, and the tubular member and screw may be rotated to provide relative rotation therebetween. Alternatively, the string may be rotated from the surface to counter-rotate the tubular member and screw via the differential gear assembly.
The material may be isolated within the borehole by providing a storage chamber within the tubular member. The storage chamber may be disposed in an annular or cylindrical cavity defined by inner walls of the member and at least partially defined by a filter. Thus, material separated from the fluid by the filter may be collected in the storage chamber, which material may be removed from the borehole by withdrawing the tubular member. Alternatively, the tubular member may have an upper inlet; fluid may be pumped into the borehole below the tubular member and circulated around the tubular member through an annulus defined between the outer wall of the tubular member and the wall of the borehole, thereby transporting entrained material up the annulus. This may create a venturi effect, such that fluid exiting the annulus above the tubular member decreases in velocity, causing the entrained material to come out of suspension with the fluid and fall into the tubular member.
Preferably also, the apparatus includes means for dislodging material from the borehole wall, which means may include a drill bit, casing scraper or end mill. Most preferably, the apparatus includes a body carrying a scraper defined on a flat of the body, the body including means for urging the scraper towards the borehole wall. Said urging means may be normally retracted, and may be extended by fluid pressure. The urging means may be in the form of one or more shoulders, circumferentially spaced from the scraper, and adapted to direct fluid towards the scrapers. The scraper may include one or more blades, with a fluid channel defined in front of each blade, such that fluid may pass upwardly through the channels. Preferably, at least two blades are provided, a leading blade defining a relatively aggressive cutting surface to dislodge and break up material, such as scale, from the borehole wall, and the following blade being less aggressive to clean the wall.